A fuel injection valve comprises a nozzle, also known as an injector tip, which is inserted into the combustion chamber of an internal combustion engine. During engine operation, this injector tip is exposed to high temperatures generated by combustion in the combustion chamber. It is not uncommon for flame temperature in the combustion chamber to exceed 4000 degrees Fahrenheit (about 2200 degrees Celsius), and in such situations the injector tip could be heated to service temperatures in excess of its tempering temperature.
For a fuel injection valve employed by a conventional diesel-fueled engine, the liquid diesel fuel serves as the cooling medium that cools the injector tip. That is, heat is transferred from the body of the injector tip to the liquid fuel that is present within the injector tip in the annular cavity around the valve needle, and the flow of liquid fuel through the injector tip and into the combustion chamber provides sufficient cooling for the body of the injector tip.
However, gaseous-fueled engines are being developed to substitute diesel fuel with cleaner burning gaseous fuels. A problem with gaseous fuels such as natural gas is that, compared to diesel fuel, much higher temperatures and pressures are needed for auto-ignition. A solution to this problem, which allows the major components of diesel engines to be preserved, injects a small amount of more auto-ignitable fuel such as conventional diesel fuel, to initiate the ignition of the gaseous fuel.
In addition, diesel fuel may be employed within the fuel injection valve to act as a lubricant and coolant. However, the quantity of pilot diesel fuel that is injected into the combustion chamber to trigger ignition of the gaseous fuel is much less than the quantity of diesel fuel that would be injected into the combustion chamber in a conventional diesel-fueled engine. During engine operation, the quantity of pilot diesel fuel may be less than 5% of the total fuel consumed (measured on an energy basis). When the diesel fuel consumption is this low, the cooling provided by the flow of liquid diesel fuel through the fuel injection valve may not be sufficient to cool the injector tip when peak combustion chamber temperatures are reached. If the injector tip overheats, possible consequences include, heat deformation of the injector tip body and fuel “coking”, a condition created by fuel being exposed to reducing conditions.
Some fuel injection valves have been proposed that divert some of the liquid fuel from the nozzle tip back to drain, thereby increasing the flow of liquid diesel fuel through the fuel injection valve to help with cooling.
For example, published German patent application DE 3928912 A1 discloses a fuel injection valve that uses fuel to cool the nozzle body by draining fuel through a drain passage that is opened when the fuel injection valve is opened by lifting the valve needle from the valve seat. With such arrangements, fuel is only returned to drain when the fuel injection valve is open, so the cooling effect of the fuel flow is only realized during the brief duration of an injection event. In addition, fuel is sent to drain whenever the fuel injection valve is open, including times when extra cooling is not necessary, such as times during start up or when the engine is idling. It is especially desirable to reduce parasitic loads during start up, when power needs may be supplied by a battery. Accordingly, a problem with this arrangement is that it can result in significant parasitic loads during start-up and idling conditions. Also, because the timing for draining fuel is indiscriminant, in that an amount of fuel is drained with every injection event, this arrangement may require a larger fuel pump than would otherwise be required.
U.S. Pat. No. 3,737,100 discloses an internally cooled unit injector that intermittently drains fuel when fuel is not being injected into a combustion chamber. The times when fuel is drained from the unit injector is controlled by the position of lands and grooved features of the plunger that respectively cover and open drain passages. With this arrangement, fuel is drained in each cycle regardless of whether the engine is starting up or already running, again resulting in significant parasitic loads at times when it is desirable to reduce parasitic loading. More importantly, during start up, the fuel pump operates at a lower speed, compared to normal operation, and increasing fuel flowrate during start up may require a larger pump, which is not necessary during normal operation, when the pump typically operates at a faster speed.
Published German patent application DE 198 47 388 A1, (the '388 Application), discloses a fuel injection system that is operable to drain a portion of the fuel from the nozzle fuel cavity. The '388 application teaches cooling the fuel injection valve during engine braking or at crankshaft angles when fuel is not being injected into the engine's combustion chamber. The fuel flow to drain is regulated dependent upon an operational parameter of the engine, such as engine RPM and/or the loading pressure generated by an exhaust turbo charger. A simple shut off valve and a variable throttle are disposed in a drain line outside of the fuel injection valve to regulate flow of fuel from the fuel injection valve through the drain line. A problem with this arrangement is the complexity of the drain valve and throttle arrangement, which adds a high pressure drain system downstream from the fuel injection valve, with joints connecting the high pressure drain line to the fuel injection valve body, the drain valve, and the throttle.
U.S. Pat. No. 5,740,782 discloses many different arrangements for cooling a fuel injection valve, including using fuel as the coolant. In one such arrangement cooling fluid passages disposed within the body of the injector tip act as a cooling jacket. The cooling fluid passages are in permanent fluid communication with the fuel supply rail for continuously draining fuel from the fuel supply system.
As noted with respect to the references already presented, continuously draining pilot fuel from the fuel supply system or the nozzle cavity increases the parasitic load on the fuel supply system and could require a larger fuel supply pump. Accordingly, there is a need to provide cooling to the injector tip, while enabling engine start up without oversizing other engine components much beyond what is required for normal operation.